A bacterial cell wall consists of linear polysaccharide chains that are cross-linked by short peptides. This arrangement confers mechanical support to the cell wall, and prevents the bacteria from bursting due to the high internal osmotic pressure. Cross linking takes place after lipid-linked disaccharide-pentapeptide constructs (lipid intermediate II) are incorporated into linear polysaccharide chains by a transglycolase enzyme. The cross-linking reaction is the last step of the synthesis of the cell wall, and is catalyzed by an enzyme known as peptidoglycan transpeptidase.
One method by which antibacterial agents exert their antibacterial activity, is by inhibiting the transglycosylase enzyme, thus interfering with the penultimate step of the synthesis of the bacteria cell wall. Although not wishing to be bound by theory, it is believed that a glycopeptide, for example vancomycin, binds with high affinity and specificity to N-terminal sequences (L-lysyl-D-alanyl-D-alanine in vancomycin-sensitive organisms) of peptidoglycan precursors known as lipid intermediate II. By binding to and sequestering these precursors, vancomycin prevents their utilization by the cell wall biosynthesis machinery. In a formal sense, therefore, vancomycin inhibits the bacterial transglycosylase that is responsible for adding lipid intermediate II subunits to growing peptidoglycan chains. This step preceeds the cross-linking transpeptidation step which is inhibited by beta-lactams antibiotics. It is likely that vancomycin also inhibits transpeptidation which involves the D-alanyl-D-alanine termini; however, as this step occurs subsequent to transglycosylation, inhibition of transpeptidation is not be directly observed.
Antibacterial agents have proved to be important weapons in the fight against pathogenic bacteria. However, an increasing problem with respect to the effectiveness of antibacterial agents relates to the emergence of strains of entrococci that are highly resistant to such agents; for example, vancomycin-resistant entrococci (VRE), which are also multi-drug resistant. It would therefore be highly desirable to find antibacterial agents that are active against a broad spectrum of bacteria, in particular resistant strains such as VRE. It would be also be advantageous to discover antibacterial agents that demonstrate high activity and selectivity toward their targets, and are of low toxicity.
We have discovered that covalent connection of two or more ligands (antibacterial agents) by a linker or linkers provides a multibinding agent that affords an improved biological effect when compared to the same concentration of unlinked ligand (i.e. in its monomeric state), or when compared to a ligand monomer coupled to the linker only. That is to say, an improved biological and/or therapeutic effect of the multibinding agent is obtained as measured against that achieved by the same number of unlinked ligands available for binding to the ligand binding site of the peptidoglycan transglycosylase enzyme substrate.
A preferred ligand is vancomycin. Particularly preferred are vancomycin bivalent compounds, which demonstrate greatly enhanced biological effect when compared to vancomycin monomer, or vancomycin monomer to which is attached the linking structure. They are also highly effective when tested against VRE strains.
Although not wishing to be bound by any particular theory or proposed mechanism of action, it is believed that the surprising activity of the compounds of the invention arises from their ability to bind in a multivalent manner with their target and thus lower the energetic costs of binding (i.e. the phenomena of energetically coupled binding), which is produced by the optimum positioning of two or more molecules of a ligand in relationship to its binding site, i.e., a multivalent interaction. That is to say, the compounds act as multibinding agents, in which ligands that are covalently attached by a linker or linkers simultaneously (or contemporaneously) bind to multiple binding sites on another component, such as an enzyme substrate.
Related Disclosures
Vancomycin derivatives are disclosed in Patent Applications EP 0 802 199, EP 0 801 075, EP 0 667 353, WO 97/28812, WO 97/38702, and in JACS 118, pp 13107–13108 (1996), JACS 119, pp 12041–12047 (1997), and JACS 116, pp 4573–4590 (1994). The disclosures of these and other documents referred to throughout this application (e.g., in the Pharmacology section of the Detailed Description of the Invention) are incorporated herein by reference.